It is, of course, annoying when, for some reason, connection to a desired user telecommunications terminal is not possible. This is particularly so with cellular phones, which are often switched off, unattended or out of range, so that resulting call completion rates for land-to-mobile traffic are low (especially at certain times of day).
Messaging services are well known; in particular, in a voice messaging service such as Voicebank (TM), a spoken message is digitally recorded in the mailbox number of the subscriber for whom it is intended, who can then retrieve by himself dialling that mailbox. However, since not every user has a mailbox, voice messaging is not of itself usually an option if a call to a desired user cannot be established. Moreover, a second, different number must be recalled and dialled by the caller, and there is also no guarantee that the called party will play the message. When a user knows that he will be unavailable, he could divert his calls to his voice mailbox, but he must remember to do so and, of course, he may not be aware that he is unavailable (for example, if he is out of range).